1. Field of Use
The present invention relates to a method for producing an optical quartz glass for use in excimer lasers. Specifically, it relates to a method for producing an optical quartz glass suitable for making an optical system of a lithographic system using an excimer laser radiation as the light source. It also relates to a method for producing a synthetic quartz glass for use in the optical system of an ArF excimer laser lithographic system, such as a large lens, prism, or beam splitter, and to a vertical furnace for carrying out the production method.
2. Prior Art
With the recent increase in the degree of integration in LSIs, the integrated circuits patterned on wafers are becoming finer, and mass production of Ultra LSIs provided with ultrafine patterns in the order of a quarter micron (0.25 xcexcm) or even finer is now under way. To obtain such ultrafine patterns, it is necessary to use exposure light sources having still shorter wavelengths, and, steppers using excimer laser radiation as the light source have been developed. Steppers equipped with KrF excimer laser radiation (248 nm in wavelength) are already in practical use, and steppers using ArF excimer laser radiation (193 nm in wavelength) as the light source is attracting much attention as a promising stepper of the next generation. As a glass material which exhibits sufficiently high transmittance in the short wavelength region of the KrF excimer laser and ArF excimer laser radiation, there can be mentioned quartz glass, fluorite, etc. As an optical material for lithography using excimer laser radiation as the light source, it is preferred to use a synthetic quartz glass prepared by vitrification into a transparent glass from a product obtained by flame hydrolysis of a high purity silicon compound and the like because it exhibits high transmittance in the short wavelength region of 260 nm or less.
However, if excimer laser radiation such as those of the KrF excimer lasers and ArF excimer lasers are irradiated for a long duration of time to the synthetic quartz glass above, paramagnetic defects known as Exe2x80x2 centers or NBOH centers (NBOHC) are generated inside the synthetic quartz glass and impair long time stability, because the excimer lasers yield high energy pulse radiation with a life of about 20 nanoseconds. As a means to solve such problems, JP-A-Hei3-88742 proposes a method of doping the synthetic quartz glass body with hydrogen. However, this method may lead to a generation of reducing defects in the synthetic quartz glass during its production process, and these defects easily cause breakdown of the glass structure upon irradiation of the excimer laser radiation and yield paramagnetic defects. The resulting defects then generate an undesirable absorption band at a wavelength of 215 nm, thereby considerably deteriorating the transmittance of excimer laser radiation, particularly, that of the ArF excimer laser radiation. The paramagnetic defects mentioned above are the oxygen deficient type defects which are generated as a result of the combination of Si in the quartz glass structure (SiO2) with a less than stoichiometric quantity of oxygen. In JP-A-Hei6-166528 a method is proposed to prevent the generation of these parametric defects from occurring, which comprises subjecting the quartz glass to an oxidation treatment in the temperature range of from 600 to 1500xc2x0 C. under an atmosphere containing oxygen, followed by a treatment in the temperature range of from 300 to 600xc2x0 C. under an atmosphere containing hydrogen. However, this method suffered a disadvantage because the treatment in the hydrogen atmosphere was performed at such a low temperature range of from 300 to 600xc2x0 C. Therefore, the diffusion rate of hydrogen into the synthetic quartz glass body was too low and required an extremely long duration of time. In particular, for the optical system with larger aperture and increased thickness, which is required with the increase in the exposure area, the method was found industrially unfeasible because the hydrogen treatment of the optical system at that temperature range required a duration of several months or longer.
However, to efficiently prevent the reducing defects that are generated during the hydrogen treatment of the synthetic quartz glass, it seems to be effective to first incorporate oxygen excessive type defects in the quartz glass followed by a hydrogen treatment. Therefore, a production method of optical quartz glass for excimer lasers was developed which comprises forming a porous silica preform by depositing silica in a soot,like form produced by flame hydrolysis of a high purity volatile silicon compound on a base body followed by heating the resulting porous silica preform to vitrify it into a transparent body and thermally heating it in a reducing atmosphere, as described in Japanese patent application Hei10-371084. However, this method is disadvantageous in that the production cost becomes high, because the optical quartz glass for use in excimer lasers is produced in two steps, i.e., by performing heat treatment in a reducing atmosphere after vitrifying the preform in an oxidizing atmosphere to obtain the transparent body.
An object of the present invention is to provide a method for producing an optical quartz glass for use in excimer lasers having excellent transmittance of and resistance against laser radiation at a low cost.
Another object of the present invention is to provide a method for producing a large optical quartz glass for use in ArF excimer lasers still having excellent transmittance and resistances against ArF laser radiation, yet at a low cost.
A still other object of the present invention is to provide a vertical heating furnace for carrying out therein the aforementioned production methods.
The objects above are accomplished by a method for producing an optical quartz glass for use in excimer lasers, comprising a step of forming a porous silica preform by depositing silica in a soot-like form formed by flame hydrolysis of a volatile silicon compound, and a step of vitrifying said porous silica preform into transparent glass in an atmosphere containing water vapor and hydrogen, as well as to a vertical heating furnace for performing therein the production method above.
Examples of volatile silicon compounds are silicon tetrachloride, methyl trimethoxysilane, tetramethoxysilane, etc. These compounds are available at high purity. By subjecting them to flame hydrolysis, SiO2-particles are deposited on a rotating heat-resistant base body to form a porous silica preform, a silica in a soot-like form (referred to hereinafter as a soot body). In the present invention, the soot body is then heated in an atmosphere containing water vapor and hydrogen for vitrification to obtain a transparent body.
The heating is preferably performed in a temperature range of from 1400 to 1700xc2x0 C., but more preferably, in a temperature range of from 1500 to 1600xc2x0 C. At a temperature lower than that range, the soot body generates melt residues, and at a temperature higher than 1700xc2x0 C., the treatment furnace undergoes deformation to bring about a dangerous state.
Concerning the atmosphere containing water vapor and hydrogen, preferably, the water vapor concentration is in a range of from 1 to 20% by volume and the hydrogen concentration is in a range of from 20 to 90% by volume. If the water vapor concentration is lower than 1% by volume, the resulting quartz glass yields a poor resistance against laser radiation, and if the water vapor concentration should exceed 20% by volume, bubbles may remain inside the resulting quartz glass. If the hydrogen concentration is lower than 20% by volume, there would be no improvement in the resistance against laser radiation, and if the hydrogen concentration should exceed 90% by volume, the transmittance of laser radiation would be impaired due to the presence of reducing defects.
As other gaseous components to be included in the above atmosphere containing water vapor and hydrogen, there can be mentioned inert gases such as He, Ar, and N2.
In vitrifying the soot body into a transparent product, it is preferred that the soot body is first densified and then subjected to vitrification to obtain the transparent body. By thus performing the densification, a synthetic quartz glass body improved in shapeability can be obtained without causing deformation during the vitrification into a transparent body. Furthermore, the densification has another advantage that it improves the optical uniformity of the synthetic quartz glass body. The preheating above is preferably performed in an inert gas or in an oxidizing gas in the temperature range of from 800 to 1400xc2x0 C.